I believe the rule is, you cannot change your username because if one was to change theirs, everyone would want to change their username.
Think carefully before you create your username.
"Sow a thought, reap an action;
Sow an action, reap a habit.
Sow a habit, reap a character.
Sow a character, reap a destiny"
"Don't let school interfere with your education."-Huckleberry Finn